1. Field of the Invention
The present invention relates to a vehicle navigation system. Specifically, the present invention relates to a vehicle navigation system, which receives radio waves from satellites and recognizes a current position of a vehicle that has the navigation system mounted thereon.
2. Description of Background Art
A GPS navigation system utilizing an artificial satellite has hitherto been used as a navigation system for various mobile bodies such as automobiles, airplanes, and ships. A GPS receiver used in this GPS navigation system is configured to receive radio waves from a plurality of GPS satellites, measure a receiving point from data of a pseudorange between each of the GPS satellites and the receiving point and a position data of each GPS satellite, and output GPS positioning data. Herein, the pseudorange contains a time offset of the receiver. However, the GPS positioning accuracy includes error components because of the following reasons:
(1). The accuracy is reduced because of an error occurring while radio waves pass through the ionosphere.
(2). The accuracy is reduced because of a multipath phenomenon in which signal radio waves from the satellites are mixed with components of signals reflected from the ground, buildings, mountains, and the like. The accuracy changes with time as the state of such obstacles changes with time.
Therefore, a simple navigation system with no autonomous navigation unit mounted thereon includes a technical problem in that a pointer on a screen indicating a current position of a vehicle that has the navigation system mounted thereon moves while the vehicle is stopped. FIG. 8 is a view showing a temporal transition of the current position obtained by GPS positioning calculation while a vehicle is stopped.
In order to overcome the above technical problem, Japanese Patent Laid-Open publication No. 63-238479 discloses a technique that prevents the current position indicator from moving while the vehicle is stopped. In this technique, current position information obtained by GPS positioning means is ignored when vehicle stop detecting means detects that the vehicle is stopped.
However, the aforementioned disadvantage due to the GPS positioning error can also occur while the vehicle is traveling at a low speed in a traffic jam or the like as well as while the vehicle is stopped. Therefore, if the current position information is ignored only when it is detected that the vehicle is stopped, wrong position information is displayed while the vehicle is traveling at low speed.
If the aforementioned technique according to the background art is not only applied to the time when the vehicle is stopped but is also extended to the time when the vehicle is traveling at low speed, a technical problem occurs in that the last determined current position greatly differs from the actual current position if the vehicle continues to travel at a low speed.